1. Field of the Invention
This invention relates to a wind turbine system and more particularly to a composite vertical axis wind turbine system that utilizes a lower wind speed vertical axis wind turbine to start the rotation of a higher wind speed wind turbine.
2. Description of Related Art
Wind energy is rapidly emerging as one of the most cost-effective forms of renewable energy with an ever-increasing installed capacity around the world. One of the widely recognized types of turbines used for electricity generation is the well-recognized Horizontal Axis Wind Turbine (HAWT). This type of turbine features a high blade tip velocity ratio, relatively high power generation efficiency, and low start-up torque. The second major group of wind turbines is the Vertical Axis Wind Turbines (VAWT), which possess several inherent advantages over HAWTs: VAWTs do not have to be yaw-adjusted to follow the changing direction of prevailing wind, and consequently handle gusts more efficiently; the power generator can be integrated into the system at ground level, reducing the structural requirements of the support tower, are much quieter in operation, lower in vibration and bird-friendly. However, a major disadvantage of most VAWT configurations is that they require a relatively high start-up torque. An omnidirectional vertical wind turbine electric generator system has been disclosed in U.S. Pat. No. 7,109,599 to Watkins. The contents of this patent are incorporated herein by reference.
Because of typical blade configurations and mechanical stiction (“static friction”) in vertical axis wind turbines, it is known that starting a vertical axis wind turbine requires a higher wind speed than is necessary to keep the turbine rotating once it is in motion. Relatively smaller vertical axis wind turbines will start at lower wind speeds such as, for example, 3 miles per hour, whereas larger-sized units would require a higher wind speed (say 8 miles per hour) to start but might continue to rotate, once having been started, at, for example, 5 miles per hour.
It is an object of the present invention to address this major deficiency of VAWT by proposing a double-vertical-axis-turbine system with a torque-amplifying cascade arrangement. This system features a small vertical axis turbine that starts at a relatively lower wind speed which, once up to speed, subsequently starts a relatively higher wind speed vertical axis wind turbine.